1. Field of the Invention
The present invention relates to a vehicle which is provided with both a traction control system and an anti-skid braking system in which a braking hydraulic pressure is adjusted to control a slip rate of a driving wheel of the vehicle.
2. Description of the Prior Art
Conventionally, there has been known a traction control in which a slip rate of a drive wheel is calculated and a braking hydraulic pressure is controlled to obtain a target slip rate of the drive wheel so that an excessive torque is not transmitted to the driving wheel. This prevents the wheel from racing, which deteriorates acceleration properties.
On the other hand, there has been also known an anti-skid braking system in which a slip rate of a driving wheel is calculated and a braking hydraulic pressure is controlled to obtain a target slip rate of the driving wheel so that an excessive braking force is not transmitted to the driving wheel. This prevents the wheel from lockings which deteriorates braking properties.
Furthermore, there has been known a vehicle, as shown in Japanese Utility Model Public Disclosure (JP, U) No. 61-183249, which is provided with both the traction control system and the anti-skid braking system.
It should, however, be noted that if the anti-skid control is started just after the traction control in the case where both the traction control system and the anti-skid control system are provided in a rear wheel drive vehicle, the braking pressure may be unduly reduced during the anti-skid braking control so that the braking control may be unduly deteriorated.
In detail, the traction control of the vehicle is generally applied independently to right and left drive wheels. On the other hand, the anti-skid control is applied in common to the right and left drive wheels so as not to lock whichever drive wheel tends to be locked.
For instance, when the left drive wheel races for some reason and the acceleration pedal is operated for accelerating the vehicle, the traction control is carried out so as to eliminate the racing of the left drive wheel by controlling the braking hydraulic pressure for the right and left drive wheels independently. At this moment, if the brake pedal is operated for braking, the traction control terminates. However, the braking hydraulic pressure partially remains in a hydraulic control system for the left drive wheel even though the traction control is finished. Then, the braking pressure is commonly increased for both the right and left drive wheels with a pressure difference because of the residual pressure applied to the left drive wheel. Thereafter, when the braking pressure is increased to a level which produces a lock condition of a drive wheel, the anti-skid control is initiated so as to eliminate the lock condition by reducing or holding the braking pressure for both drive wheels commonly. In this case, even though the braking pressure for the right wheel does not reach a level which is high enough to produce the locking condition, the braking pressures for the right drive wheel and for the left drive wheel are reduced. Since the pressure difference between the right and left drive wheels still remains and the anti-skid control is applied commonly to the right and the left drive wheels, the braking pressure for the right drive wheel may not be enough to provide a desired braking effect.